Throughout the history of drilling and servicing oil wells, accidents, some even involving fatalities, frequently occur when a rig is pulling tubulars and runs the tubulars into a wellhead, BOP, slips, or other stationary apparatus. Normally, when the rig is pulling shallow and has a light load, the block speed is fast and there is little time to react to unexpected occurrences. Compounding the speed problem, there is no forgiveness or stretch in tubing or drillpipe, and thus, when the pipe hangs up, damage and accidents frequently occur.
When a rig starts pulling tubulars out of a wellbore, the operator or driller will select the most efficient gear to pull the load based on the weight of the hookload and the desired speed of the pull. More often than not the operator pulls as fast as possible, however the block pulling speed is limited by both the prime mover (engine) horsepower and the gear train transferring the power from the engine to the hoist. Since hookloads coming off bottom of a hole can be high, the normal rig with the normal operator will start off bottom pulling slowly (engine maxed out but still slow block movement) and as the hookload decreases with less pipe in the hole, the pulling speed or block velocity will increase. This is primarily due to the reduced horsepower requirement to lift the lighter load.
Drillpipe, tubing, and rods all have a known modulus of elasticity and exhibit the ability to stretch. For example, if a rig has 10,000 feet of surface measured tubing that weighs 45,000 pounds, and the tubing is not moving, the weight indicators will sense 45,000 pounds provided the pipe is hanging free and is vertical. The bottom of the tubing will be at approximately 10,003 feet due to normal free hanging stretch.
When subjected to forces in excess of the free hanging weight, such as being getting stuck in the hole where the bottom of the tube is stationary and the top of the tube being pulled by the block is moving, the tubular string will elongate. The amount of additional stretch can be defined with the following equation:
            1      )        ⁢                  ⁢    Stretch    ⁢                  ⁢          (      in      )        =                    [                  Length          ⁢                                          ⁢          of          ⁢                                          ⁢          pipe          ⁢                                          ⁢          in          ⁢                                          ⁢          hole                ]            *              [                  Differential          ⁢                                          ⁢          Pull                ]                    [              735        ,                  000          *                      [                          Weight              ⁢                                                          ⁢              of              ⁢                                                          ⁢              pipe                        ]                              
When a tubular gets stuck deep in a hole, the operator's reaction time is significantly longer than when a tubular gets stuck near the surface. For example, if a 2⅜″ tubular at 4.5 pounds per foot gets stuck at 10,000 feet, the free hanging weight is 45,000 pounds. The maximum desired pull would then be 65,000 pounds, which is based on a calculated value based on the 90% of yield point of new tubing. If the rig operator were to pull an additional 20,000 pounds over the free hanging weight (i.e. the 65,000 pound maximum), the total stretch in the tubing would be:S=[10,000 feet*20,000 pound over pull]/[735,000*4.5 #/ft]=60 inches  2)
Using this equation and applying it to the rig pulling out of the hole, a determination can be made as to what the operator sees when the pipe sticks while being pulled. Assuming the rig's pulling speed is, at that depth and weight, about 60 feet per minute or one foot per second, and knowing from equation 2 that a 20,000 pound over pull yields a 60 inch stretch, the time that is taken from sticking to a 20,000 pound over pull can be calculated as follows:T=D/V  3)Where D is the distance pulled, V=velocity and T is time. Knowing that at a 20,000 over pull the tubular stretches 60 inches, or 5 feet, and that the velocity at that depth is 1 foot per second, time can be calculated as follows:T=5/1 or five seconds  4)In other words, if the tubing sticks near the bottom of the hole, the operator has approximately five seconds to react and stop the blocks prior to reaching the maximum allowable pull of 65,000 pounds, i.e. a 20,000 pound overpull. Of course, the greater the speed, the less reaction time is afforded the operator, however overpull is normally quickly noticed by the operator, and therefore the operator usually has time to shut the rig down and take evasive action to avoid and overpull.
Comparing the deep hole sticking example to a shallow sticking example magnifies the problem facing the drilling and well servicing industry. Assume that the same tubular is at a depth of only 500 feet, the tubular has a hanging weight of 2,250 pounds, at the same 4.5 pounds per foot. Now the rig operator has more than enough horsepower to pull at almost any speed, but still does not want to pull more than 65,000 pounds, which in this case is a 62,750 pound overpull. Using equation 1, the tubular stretch in this example is calculated as follows:S=[500 feet*62,750 over pull]/[735,000*4.5 #/ft.]=9 inches  5)Assuming the pulling speed for a light load is fast at four feet per second, using equation 3 the time for this “almost out of the hole” example can be calculated as follows:T=0.75/4=0.1875 seconds to react.  6)As shown, the time to travel the 9 inches, i.e. to stretch from hanging weight to maximum pull (¾ of a foot), at 4 feet per second is 0.1875 seconds, significantly slower than when the tubular gets stuck deep in the hole. Even if the rig is operated at a much lower gear and the pulling speed is slowed to 1 foot per second, using equation 3 again to calculate time, it can be shown that there is still not enough time (¾ of a second) to properly react to a shallow sticking situation:T=0.75/1 =¾ second to react.  7)
As shown, when the rig has hole problems that cause sticking, if there is an ample length of tube in the hole, the operator has a sufficient amount of time to react. If, on the other hand, the length of the tube is short and the rig is being operated at its maximum capacity, there is little or no time to react and the chances of a catastrophic event greatly increase. It remains therefore incumbent to find a solution to this problem to provide the rig and crew an extra level of safety to prevent such catastrophes.